Selenium-sulfur photoconductive target

ABSTRACT

A photoconductive device made of a mixture of sulfur and selenium in two amorphous forms one less transparent than the other comprising a mass of small particles of one form deposited in the interstices of the other.

United States Patent 72] Inventor Stanley A. Bynum Dallas, Tex.

[21] Appl. No. 807,536

[22] Filed Mar. 17, 1969 23 1 Division of sr. Noiilspizfibr. 1, 1 968, abandoned, which is a continuation of Ser. No. 467,268, June 28, 1 965, abandoned.

[45] Patented Oct. 12, 1971 i [73] Assignee General Electrodynamics Corporation Garland, Dallas County, Tex.

[54] SELENlUM-SULFUR PHOTOCONDUCTIV E TARGET 3 Claims, 1 Drawing Fig.

[52] U5. Cl SIS/65A, 252/501 [51] Int. Cl 1. H0lj 31/28 [50] Field of Search 313/65 A, 97, 94, 65 A, 65 T; 117/201, 230; 96/15; 252/500, 501

[56] References Cited UNITED STATES PATENTS 3,106,488 10/1963 Lubszynski 313/65 X 2,662,832 12/1953 Middleton et aL. 117/230 3,249,783 5/1966 Santilli et a1. 313/65 A FOREIGN PATENTS 1,027,841 4/1966 Great Britain 313/65 1,091,869 11/1967 Great Britain 313/65 Primary Examiner-Roy Lake Assistant ExaminerV. Lafranchi Attorneys-Ned L. Conley, Murray Robinson, Robert W. B.

Dickerson and Bill B. Berryhill ABSTRACT: A photoconductive device made of a mixture of sulfur and selenium in two amorphous forms one less transparent than the other comprising a mass of small particles of one form deposited in the interstices of the other.

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r a I I I I I SELENIUM-SULFUR PHOTOCONDUCTIVE TARGET CROSS-REFERENCE TO RELATED APPLICATIONS BACKGROUND OF THE INVENTION This invention relates to photoconductive devices, and more particularly it relates to photoconductive image pickup tubes of the type as the vidicon.

Television camera tubes employing photoconductive targets and known as vidicons are now well known in the art, having been described in an article in the May 1950 issue of Electronics magazine, and in a number of patents including, for example, U.S. Pat. No. 2,745,032 to Forgue, et al. As described in the prior art, and as is well known, a vidicon camera tube consists of an electron gun and a target assembly contained in a glass envelope, usually about 6 inches long and 1 inch in diameter. The electron gun may be of the conventional type used in other types of television pickup tubes. The target assembly comprises a film of light-transparent, electrically conductive material on the face plate of the envelope, and a coating of photoconductive material deposited upon the electrically conductive film. The target and the gun are so arranged within the envelope that the electron beam from the gun scans the photoconductive surface of the target.

Photoconductive materials used for vidicon targets are electric insulators in the dark, but become electrically conductive when light is shined upon them. The conductivity is proportional to the amount of light striking the material, and is limited to the immediate area under the influence of the light. A number of different photoconductive materials are usable for various electronic devices, and one of the most widely used photoconductors is elemental selenium, in which, in fact, the phenomenon of photoconductivity was first observed, in 1873. Selenium has two common allotropic forms, both of which are widely used as photoconductors. The crystalline gray form, sometimes referred to as the metallic form, is a fair conductor even in the dark, and is used principally in photocells and rectifiers, and the like. The amorphous red form is used as a photoconductor for photoconductive television image pickup tubes and thin film detectors because, as noted in U.S. Pat. No. 2,654,853, it is characterized by a dark resistivity sufficiently high (approximately 10 ohm-cm.) to permit charge storage operation. It is the latter allotropic form to which the invention described herein relates.

A major difficulty in the use of amorphous red selenium in photoconductive television image pickup tubes is that the material is unstable and will slowly convert to the more conductive crystalline form even at ordinary room temperature. At temperatures above about 45 C. the time required for such conversion is only a few hours. Thus where amorphous selenium has previously been used in television camera pickup tubes, it has been necessary to strictly limit the operating temperature of the tube.

SUMMARY OF THE INVENTION It is an object of this invention to provide a selenium base photoconductor which does not have the thermal instability of red amorphous selenium photoconductors previously used.

It is another object of this invention to provide a photoconductive coating, such as is used on a vidicon camera tube face plate, which is formed principally of amorphous selenium, but which has a high photosensitivity and is far more stable at relatively higher temperatures than is ordinary red amorphous selenium. Still another object of the invention is to provide a method for forming a photoconductive surface from selenium which provides a photoconductive surface having high resistivity in the dark (at least about 10" ohm-cm.) and good sensitivity combined with a long life at ordinary operating temperatures.

Yet another object of the invention is to provide a vidicon camera tube which can be scanned at relatively long intervals and will still give good current to light response, and which has a long, useful life.

These and other objects of the invention are accomplished by forming a photoconductive surface from red amorphous selenium which has been combined with a small proportion of sulfur in such a manner that the selenium and sulfur form a complex, or an intermolecular compound, whereby the sulfur modifies the reaction of the selenium to temperature and makes the amorphous selenium more stable.

BRIEF DESCRIPTION OF THE DRAWING For a better understanding of the invention, reference is now made to the following description and to the single FIGURE of the accompanying drawing, which shows a longitudinal and partly sectional view of one form of vidicon camera tube embodying the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawing shows a vidicon type camera tube, indicated generally by the reference numeral 10, which comprises an evacuated envelope 12 having an electron gun 16 in one end thereof. The electron gun 16 may be any of the known types of electron guns, and produces an electron beam directed toward the target electrode 18 in the other end of the envelope 12. The electron beam is focused and scanned over the exposed surface of the target electrode by any conventional means (not shown).

The target electrode 18 is attached to a metal ring 19, made of a metal such as Kovar, which is sealed by means well known in the art to the edge of the target electrode and to the end of the envelope 12. The target electrode comprises a transparent face plate or substrate 20, preferably made of fused quartz or some similar material which has a low coefficient of expansion. The face plate has applied thereto a layer 22 of a conductive material, which comprises a signal plate which in turn is covered by a layer 24 of a photoconductive material.

The transparent conductive layer or signal plate 22 may be made of tin oxide or a thin evaporated metal film such as gold, and may be deposited by any known technique such as spraying or evaporation. The signal plate should be highly electrically conductive and should be transparent to the particular radiant energy for which the device 10 is designed to respond.

The photoconductive coating 24 is made up of a particular form of red amorphous selenium which is stabilized by the combination therewith of a comparatively small amount of sulfur. Such a coating is prepared by first combining sulfur and selenium in such a way that an amorphous atomically intimate and homogeneous mixture of selenium and sulfur is obtained. Such a mixture may be obtained, for example, by comelting the two elements in a protective atmosphere, or by coprecipitation, or by coevaporation of the two elements. It has been found that comelting of the two elements in an evacuated tube at a temperature of around 400 C. is a convenient and reliable method of obtaining the desired homogeneous mixture.

After the mixture of selenium and sulfur is prepared it is fabricated into a photoconductive thin film lying on the signal plate of the target electrode. Such a thin film may be prepared by means well known in the art, such as, for example, by evaporating either in a high or low vacuum, or by settling, or by electrophoretic plating, or by other well-known methods for applying such photoconductive layers. Such methods of depositing such coatings are well known in the art and do not form a part of this invention. However, good results have been achieved in forming the coating of this invention by evaporating the selenium sulfur mixture onto the conductive layer at a temperature of about 300 C. in a vacuum of approximately 0.1 mm. absolute pressure.

The photoconductor as thus formed comprises a red coating, amorphous by X-ray diffraction, which has good photoconductive sensitivity and has better heat stability than ordinary red amorphous selenium. However, the characteristics of the coating are substantially improved by following the coating process with a heat treatment which comprised holding the photoconductor at a temperature in the range of about 40 C. to about 100 C. for a time sufficient to convert the initial amorphous form to a second form which is also amorphous by X-ray diffraction, but which is characterized by a darker red color and a lower transparency than the original form. This second form of selenium-sulfur compound has been found to possess a higher photosensitivity than the first form, and also has excellent resistance to conversion into the gray crystalline form under exposure to moderate tempera ture. The precise time required for the conversion from the first amorphous form to the second amorphous form will vary according to the particular temperature selected. It has been found that if the heat treatment is carried out at about 70 C., heat treatment for 1 hour is sufficient to obtain substantial conversion to the second amorphous form.

The final product achieved by this process appears to be a mass of very small spherical particles of the second amorphous form with a small amount of the first amorphous form filling the interstices. The fact that the amorphous form obtained by the process of this invention is different from the usual red amorphous form is made evident by the performance of the photoconductive material. Vidicons which previously used red amorphous selenium for the photoconductor were limited to operations at temperatures under 45 C., and even at this temperature had a life of only a few hours before the red amorphous material would begin to convert to the gray crystalline form, which has such a low dark resistivity that it cannot be used in a vidicon. However, a vidicon which is provided with a photoconductor of the type prepared according to this invention has a much longer life even though operated at temperatures as high as 70 C. or greater. Apparently the addition of the sulfur to the selenium stabilizes the amorphous form of the selenium and prevents its ready conversion to the gray form upon heating.

it is not known precisely what proportion of sulfur is present in the coating applied to the signal plate of the target. However, it is thought likely that the proportions of the materials in the photoconductive layer are very near the proportions which are in the mixture which is evaporated. It has been found that the advantageous results of this invention are obtained when the mixtures evaporated in forming the photoconductive layer comprise sulfur amounting to from about one-half percent to about 20 percent of the weight of the selenium present, although a preferred amount of sulfur is between about 5 percent to about percent of the weight of selenium.

Various embodiments of the invention are shown in the drawing and described in the specification, but many variations thereof will be apparent to those skilled in the art. It is not practical to show or describe all the variations included within the invention, and therefore the embodiments described should be considered illustrative only, and not limiting, the scope of the invention being as broad as is defined by the appended claims. The form of the claims and the specification, including the Abstract, is adopted solely for easier reading and understanding, and should not be considered in interpreting the scope of the invention claimed.

1. Apparatus comprising an evacuated envelope, a target in one end of said envelope, and an electron gun in said envelope for producing an electron beam to scan said target, I said target comprising a transparent conductive layer, and a photoconductive layer contacting said conductive layer consisting essentially of t a complex of sulfur and selenium in which the weight of sulfur is from about one-half percent to about 20 percent of the weight of selenium, in two amorphous forms, the secon form of which 18 less transparent than the first form, and comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass. 2. Apparatus comprising a conductor, and a photoconductive layer contacting said conductor, said photoconductive layer consisting essentially of a complex of selenium with from about one-half percent to about 20 percent by weight sulfur, in two amorphous forms, the second form of which is less transparent than the first form, and comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass. 3. A photoconductive target for a television camera tube comprising a face plate having a low coefficient of expansion, a transparent conductive coating on said face plate, and a photoconductive coating on said conductive coating, said photoconductive coating consisting essentially of a complex of sulfur and selenium in which the weight of sulfur is from about one-half percent to about 20 percent of the weight of selenium, said complex being in two amorphous forms, the second of said amorphous forms having less transparency than the first, and having higher resistance to conversion to a crystalline form at temperatures up to about 70 C., and said coating comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass. 

1. Apparatus comprising an evacuated envelope, a target in one end of said envelope, and an electron gun in said envelope for producing an electron beam to scan said target, said target comprising a transparent conductive layer, and a photoconductive layer contacting said conductive layer consisting essentially of a complex of sulfur and selenium in which the weight of sulfur is from about one-half percent to about 20 percent of the weight of selenium, in two amorphous forms, the second form of which is less transparent than the first form, and comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass.
 2. Apparatus comprising a conductor, and a photoconductive layer contacting said conductor, said photoconductive layer consisting essentially of a complex of selenium with from about one-half percent to about 20 percent by weight sulfur, in two amorphous forms, the second form of which is less transparent than the first form, and comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass.
 3. A photoconductive target for a television camera tube comprising a face plate having a low coefficient of expansion, a transparent conductive coating on said face plate, and a photoconductive coating on said conductive coating, said photoconductive coating consisting essentially of a complex of sulfur and selenium in which the weight of sulfur is from about one-half percent to about 20 percent of the weight of selenium, said complex being in two amorphous forms, the second of said amorphous forms having less transparency than the first, and having higher resistance to conversion to a crystalline form at temperatures up to about 70* C., and said coating comprising a mass of small particles of the second form having a small amount of the first form in the interstices of said mass. 